It is generally known that the amount of hydrocarbons, carbon monoxide and oxides of nitrogen emitted through operation of an internal combustion engine may be substantially reduced by controlling the relative proportion of air and fuel (air/fuel ratio) admitted to the engine, and by catalytically treating the engine exhaust gas. A desirable air/fuel ratio is the stoichiometric ratio, which is known to support efficient engine emissions reduction through the catalytic treatment process. Even minor deviations from the stoichiometric ratio can lead to significant degradation in catalytic treatment efficiency. Accordingly, it is important that the air/fuel ratio be precisely regulated so as to maintain the actual engine air/fuel ratio at the stoichiometric ratio.
Closed-loop control of internal combustion engine air/fuel ratio has been applied to drive the actual air/fuel ratio toward a desired air/fuel ratio, such as the stoichiometric air/fuel ratio. This control benefits from an estimate of actual engine air/fuel ratio, such as from an output signal of an oxygen sensor disposed in the engine exhaust gas path. The estimate is applied to a control function responsive to air/fuel ratio error, which is the difference between the estimate and the desired air/fuel ratio.
The oxygen sensor may be a conventional zirconia oxide ZrO.sub.2 sensor which provides a high gain, substantially linear measurement of the oxygen content in the engine exhaust gas. A lean engine air/fuel ratio corresponds to a ZrO.sub.2 sensor output signal below a predetermined low threshold voltage and a rich engine air/fuel ratio corresponds to an output signal above a predetermined high threshold voltage.
ZrO.sub.2 sensors are disposed in the exhaust gas path in position to measure the oxygen content of the engine exhaust gas, such as upstream of the catalytic treatment device (catalytic converter). Such pre-converter sensors have contributed to success in engine emissions reduction efficiency. However, certain effects, such as sensor or converter aging (catalyst depletion) and sensor contamination may degrade emission reduction efficiency and may be left uncompensated in traditional closed-loop control.
ZrO.sub.2 sensors may also be positioned in the engine exhaust gas path downstream from the catalytic converter. For example, post-converter sensors have been applied for converter diagnostics, or for outright engine air/fuel ratio control.
Post-converter sensors are in position to provide information on the emission reduction efficiency of the air/fuel ratio control system including the pre-converter sensor and the catalytic converter. Accordingly, it would be desirable to apply information from a post-converter oxygen sensor in engine air/fuel ratio control responsive to a pre-converter oxygen sensor so as to compensate any degradation in the efficiency of the control to reduce undesirable engine emissions.